I know you all will see this one coming, but I hereby nominate... Clan Of The Cave Bear series. So much promise, and it just got so bad. So, so bad. I really think the author just quit caring and washed her hands of the whole thing halfway through Shelters Of Stone.

I prefer to think of it ending with Plains of Passage. Shelters of Stone was bad, but Painted Caves was just terrible fan fiction.

The Dune series did that for me... I loved the first book so much as a teen I went out and hunted down all the others (pre e-readers through second hand stores)... and I don't think I ever actually read the last one.

The old Gor books by John Normal would also qualify. The first few were pretty decent pulp adventure. Then he started in with his "women are naturally meant to be slaves to dominant men" philosophy. It was tame by today's standards but was pretty much the Anita Blake of its time. They were still pretty good for awhile, though. Especially back then I had no problem at all with pulp adventure spiced up with a bit of light porn. Occasionally the characters would veer off into idiotic philosophical discussions, but you could just skip over those parts. But then the pulp adventure got formulaic and the porn and philosophy took over, mostly the philosophy, so I stopped reading.

I've never yet read any of the Gor books -- though from time to time, consider doing so. I've been a bit put off, for a highly ridiculous reason. I first heard of them in a discussion on another board; where a poster referred to the people of Gor, as "Goreans". For some reason, my mind leapt to an "our-world" parallel, and I envisaged the books being about the two polar-opposite nations of South and North Gorea, which hate each other's guts, and which enmity holds possible wider implications for the planet... further reading of the thread revealed that the Gor thing was about very different stuff; and somehow, I felt cheated of my wrong interpretation, and didn't want to investigate further.

The Cat Who . . . mysteries. The first four were excellent. The next four or five (written several years later) were pretty good. Gradually, one got the feeling one was reading the same book over and over.

The Anita Blake novels by Laurell K Hamilton. The first 8/9 books were awesome. When Narcissus in Chains came around, it turned into just porn.

Piers Anthony's Xanth series. Became really repetitive.

Stephanie Plum. I still love them for fluff reading, but there's been NO plot advancement in at least 5 books. It's always about whether she'll pick Ranger or Morelli...just pick one already!

I got the sense, in the latest Stephanie Plum, that the next one will be the last in the series. Nineteen sounded to me like Stephanie is maturing, reaching a point where she's not completely incompetent at her job All.The.Time, and she's getting to the point where she's ready to pick one and settle down. I'm guessing Janet Evanovich is getting tired of finding new and creative ways for Stephanie to lose a bail-jumper, and thinks twenty is a good number to wrap up on.

Terry Goodkind's "Wizard's First Rule" was wonderful. Truly an outstanding novel from an author I had never heard of or read before. The second and third books were pretty good. After that he managed to completely lose my interest. Too much preaching, too little storytelling.

I'm glad to see I'm not the only one who got tired of Wheel of Time, Goodkind's Sword of Truth and the Anita Blake series. Most of the people I know who read them thought I was a horrible person for saying each of those series should've ended around book 4 or 5 (or in the case of the Sword of Truth, book 2 or 3).

Dragonriders of Pern was good until her son started writing, the quality plummeted fast.

Edit: And not a series per say, but John Ringo should keep his rabid politics out of his fiction. His early stuff was good, then he started to let his politics take over his stories and suddenly he sounded like some of these bat poo insane talk radio guys.

Terry Goodkind's "Wizard's First Rule" was wonderful. Truly an outstanding novel from an author I had never heard of or read before. The second and third books were pretty good. After that he managed to completely lose my interest. Too much preaching, too little storytelling.

I had the same experience with this series. Totally lost interest.

For the later ones I had more 'hit or miss' going on. Definitely too much preaching/harping going on as they went further, but one would be unreadably awful and another would have enough story to compensate. Almost worse than a completely awful downhill slide, because there was still a chance it might be decent, so I'd have to try it...

The Anita Blake novels by Laurell K Hamilton. The first 8/9 books were awesome. When Narcissus in Chains came around, it turned into just porn.

Piers Anthony's Xanth series. Became really repetitive.

Stephanie Plum. I still love them for fluff reading, but there's been NO plot advancement in at least 5 books. It's always about whether she'll pick Ranger or Morelli...just pick one already!

I got the sense, in the latest Stephanie Plum, that the next one will be the last in the series. Nineteen sounded to me like Stephanie is maturing, reaching a point where she's not completely incompetent at her job All.The.Time, and she's getting to the point where she's ready to pick one and settle down. I'm guessing Janet Evanovich is getting tired of finding new and creative ways for Stephanie to lose a bail-jumper, and thinks twenty is a good number to wrap up on.

The last 2 have hinted that Stephanie's going to choose in her next book. It hasn't happened yet. I won't be waiting with bated breath until she does, either.

Personally, I think Janet is going to keep writing them until she starts to lose fan base.

I thought that it started out pretty well, but by the time I hit the 5th or 6th book I felt like I was being punned to death. Someone gave me one that I hadn't read, and I figured I'd give it a shot since I haven't picked up a Xanth book in years. I made it through about 20 pages.

OTOH, I really liked his Incarnations of Immortality series (except the last book), and I still pick them up and reread them occasionally.